1. Field of the Invention
The present invention relates to laser apparatus and methods, and particularly relates to techniques for discriminating against higher order modes.
2. Description of the Prior Art
Techniques have been developed in the past to suppress higher order modes in a laser beam, in order to control the Gaussian energy distribution across the lase1 beam. One technique for achieving higher order mode discrimination is the utilization of a laser cavity having end mirrors spaced as far apart as is feasible, and by utilizing an aperture in the resonator to limit the beam passing between the end reflectors of the cavity to a small dimension in the center of the beam where the energy of the lower order modes is concentrated. While this technique is effective for eliminating higher order modes, the result is a significant reduction in the amount of energy that passes back and forth through the laser medium between the end reflectors to stimulate continued laser oscillation.
There have been suggestions in the prior art for utilizing optical elements in a laser cavity in order to control mode configuration, divergence and collimation. In such prior art arrangements, a positive lens element and a negative lens element are positioned along the laser energy beam path between the two end reflectors, but with the laser medium located outside of the divergent beam path between the two lens elements.
An article entitled "Stability and Beam Divergence of Multimode Lasers With Internal Variable Lenses" by Kortz et al in Applied Optics, Volume 20, Number 23, December 1981 discloses various laser resonator configurations utilizing internal lens combinations.
It is also known in the prior art to utilize curved end mirrors which achieve some degree of non-linear amplification in the laser medium during successive passes of the reflected beam through the lasing medium.